A conventional phase-locked loop (PLL) shown in FIG. 1 contains a phase detector or comparator 1 which compares the phase of an incoming signal with that of a locally generated signal, a lowpass loop filter 2 which smooths the phase comparator output to produce an error signal, and a voltage-controlled oscillator (VCO) 3 whose frequency is a function of the applied error signal voltage. Two major applications for phase-locked loops are recovering clean signals from noisy ones and demodulating FM signals from their carriers. The present invention is concerned primarily with the second one. Typical applications of FM demodulation are tone decoding, carrier frequency acquisition and carrier tracking.
Nominally, the process of demodulation is the extraction of the modulation information from the modulated carrier. Whatever demodulation process is used, a certain amount of the carrier is bound to remain. When the carrier frequency is decreased, the problem of separating this residual carrier from the desired output becomes more difficult unless the bandwidth of the modulating signal is also decreased.
There are two kinds of phase detectors used in phase-locked loops: one designed to be driven by analog signals (e.g., a four-quadrant multiplier) and the other designed to be driven by digital transitions (e.g., edge timing between the incoming signal and a VCO output signal). Horowitz and Hill, The Art of Electronics, Cambridge University Press, Second Edition, 1989, pp. 644-645. The first type, also known as a balanced mixer, produces an output voltage after filtering that is a function of the phase difference. This may be applied to the VCO as a control voltage. The second type of phase detector, sensitive to only the timing of the edge of the reference frequency cycle as compared to the edge of the input frequency cycle, is generally more complex.
As an illustration of the nature of the filtering problem, note that with the typical multiplying-type phase detector, the lowest frequency component of the phase detector output is at twice the carrier frequency, f.sub.o. However, the amplitude of this component is large, regardless of the amplitude of the modulating signal, because this kind of phase detector switches between two extreme states. Typically, the signal applied to the low-pass loop filter is a squarewave with peak amplitude equal to the local power supply voltage. Stability considerations and simplicity of calculation dictate the use of simple lowpass or lag-lead filters inside the loop. The filter of a wideband loop removes little of the energy of the signal at 2f.sub.o. For an undistorted output, a post-detector filter 4 of considerable quality is required.
When a phase-locked loop is used in a communication system, the phase detector and lowpass filter determine the lock range, the capture range and the noise performance of the loop. If the incoming signal has a large signal-to-noise ratio, there is less or no concern about noise performance. When used in the system as an FM demodulator, the loop filter has to be a compromise between flat frequency response and rejection of phase detector noise. As spectral frequency of the incoming carrier approaches the modulation frequency, this compromise becomes difficult to resolve. This invention addresses that problem.